1. Field of Invention
The present invention relates to mobile wireless devices as used in personal and asset location systems. In particular, the present invention relates to a method and apparatus for utilizing the Global Position System (GPS) to locate objects over a large geographic region and to provide information or services related to a real time position of the object.
2. Description of the Background Art
With the advent of the Global Positioning System (GPS), there is growing demand for mobile devices that can locate and track children, the elderly, emergency situations, tourists, security, valuable assets, and the like. Devices built using conventional GPS receivers have been developed by a number of companies. These current generation of devices have major limitations in terms of indoor penetration, power consumption, accuracy, and acquisition time.
To address the above issues new GPS processing architectures have evolved that utilize a combination of mobile GPS receivers and fixed GPS infrastructure communicating via wireless links. Systems with this architecture collect the majority of the data for location using the fixed infrastructure and, compared to traditional GPS, are able to offer large improvements in accuracy, indoor penetration, acquisition time, and power consumption. Thus far, such systems are based upon a fixed site GPS receiver that is physically located in the local vicinity of the mobile receiver and are therefore difficult to extend to broad coverage areas without a large proliferation of fixed site GPS receivers. Furthermore, such systems require a wireless link which provides communication in real-time and therefore such systems cannot take advantage of non real-time messaging systems such as paging networks.
Thus there is a need for a GPS processing architecture and device technology that provides the benefits of improved accuracy, indoor penetration, acquisition time, and power consumption and also offers the capability to function over large geographic coverage areas without requiring a fixed site GPS receiver in the local vicinity of the mobile device. Furthermore, to take advantage of broad coverage messaging systems, the architecture should have the ability to operate over a link which is not realtime, i.e. a link where there is significant and possibly unknown message latency.
The invention provides a method and apparatus for locating a mobile device over a broad coverage area using a wireless communications link that may have large and unknown latency. The apparatus comprises at least one mobile device containing global positioning system (GPS) processing elements, a GPS reference network comprising a plurality of fixed site GPS receivers at known locations, a position server with software that executes GPS processing a wireless communications link, and at least one location requester.
The method consists of using GPS measurements obtained at the fixed site GPS receivers to build a real time model of the GPS constellation which includes models of satellite orbits, satellite clocks, and ionosphere and troposphere delays. The model is used by algorithms within the position server to create an initialization packet used to initialize GPS processing elements in the mobile devices. Once initialized, the GPS processing elements detect and measure signals from the GPS satellites. The measurements made are returned to the position server, where additional software algorithms combine the information with the realtime wide area model of the GPS constellation to solve for the position of the mobile device. The computed position is then provided to the location requestor.
The system design is such that messages to and from the mobile device can be delayed in time by an unknown amount as would be the case for a non real-time communication system. Furthermore, only a small number of fixed site GPS receivers are required in the system and there is no requirement to have a fixed site GPS receiver in the local region of the mobile device.
The GPS processing elements in the mobile devices include a highly parallel GPS correlator that is capable of searching and detecting signals over a wide range of unknown signal delays. The highly parallel nature of the GPS processing allows the system to use long averaging periods which are impractical for a conventional GPS receiver that searches for signals sequentially using a small number of correlators. The long averaging times are made possible by the parallel correlation that allow the system to locate objects in difficult signal environments, such as inside buildings, where conventional GPS cannot function.
Furthermore, the system design is such that the GPS processing elements in the mobile devices are responsible for making only an instantaneous measurement of the sub-millisecond PN code phases of the received signals, and do not collect GPS navigation data or time tag information. Conventional GPS receivers, by contrast, require a long time period (typically one minute or more) of continuous and strong signal reception in order to acquire navigation data and timing. Thus, the system produces fixes much more quickly than a conventional GPS and can do so in environments where signals are relatively weak and unstable.
The overall architecture supports many different user models. Specifically, the location requestor may be the user of the mobile device or a different entity such as a Internet terminal or an emergency assistance center.